To increase the economic viability of transporting freight, port authorities invest significant capital into infrastructure to facilitate the efficient loading and unloading of containers from ships at dock. The infrastructure is geared to move said containers to storage facilities ready for delivery via land transport or directly from ships to the land transport.
Recently, to increase efficiency and therefore economic viability, inventions facilitating the movement of several containers simultaneously have been developed. One such invention is disclosed in WO 01/98195, the contents of which are incorporated herein by reference. This invention incorporates a spreader having connection to two container engagement frames, which may engage with one or two containers. It follows that this will immediately increase the rate for which a ship may be loaded or unloaded, and therefore the efficiency of the process.
A disadvantage of this invention is the connectivity between the spreader and engagement frames. As shown in the various figures of WO 01/98195, the engagement frames are connected to the spreader either by chains or pin-jointed tie rods.
If this arrangement were to pick up one container only or two containers of substantially varying weights, then a redistribution of the weight would lead to the spreader listing to one side due to the unbalanced force. Thus, this arrangement is restricted to lifting containers of substantially similar weights. In the loading and unloading procedure however, the only time an operator will be assured of lifting two containers of similar weight will be when the containers are empty. Thus whilst this invention is useful in moving multiple containers, in a practical sense, it may be restricted to the safe movement of empty containers only.
This development has been built on by the invention of WO 03/104132, the contents of which are incorporated herein by reference. This invention is an improvement over first in that the connection between the spreader and the container engagement frames is releasably fixed, so removing the degrees of freedom inherent with the use of the chain or pin-jointed tie rods of the previous invention. Further, to prevent the spreader frames from listing, the invention WO 03/104132 provides a hydraulically powered assembly capable of separating the engagement frames, so that adjacent containers do not interfere with each other and are therefore held in a fixed relation to each other. This has the advantage of maintaining a degree of control over the load, which is not possible with the invention of WO 01/98195.
Nevertheless, the invention of WO 03/104132 is still subject to existing infrastructure and therefore any imbalance of load through a mismatch of weights of the lifted containers may lead to the same rotation at the point at which the cables engaging the spreader to the hoist are joined. Thus whilst the spreader and engagement frames are stable, the cable would still permit rotation as the unbalanced load is redistributed. As with the previous invention, the redistribution and consequential rotation of the spreader frame may lead to difficulties in placing the containers either on the dock or on a ship.
It would be advantageous to have a system whereby containers having a weight differential can be lifted without suffering the detrimental effects of a redistribution of load.